Predicting change: Will morphological constraints on hydraulic function limit acclimation of subantarctic plants to a warmer climate?

Major changes are occurring in mid to high altitude vegetation on Macquarie Island where widespread death of plants has been observed in recent years. It is of critical importance to understand this signal from an island sentinel of climate change. The functional morphology of sub-Antarctic species was studied in relation to desiccation tolerance. The results revealed hydraulic traits that may constrain the ability of many sub-Antarctic plant species to adjust to warmer, drier conditions, thereby contributing to climate-driven change in vegetation. These results contribute to identifying species most at risk from climate change, and developing predictive models of vegetation change together with management plans to address changing climate scenarios.

Novel insight was gained on mechanisms that underpin the structure and function of subantarctic vegetation in relation to climatic variables, and how those attributes may affect responses of this unique vegetation to climate change. For example, detailed analyses of hydraulic structure and function in two cushion plant genera, Colobanthus and Azorella, revealed differences in vulnerability to drought with Azorella being the more sensitive species consistent with differences in habitat. The results provide novel evidence of a hydraulic basis for the cushion plant growth form, with the particulars of Azorella (ie a strong reliance on direct, non-vascular water uptake by the shoots combined with extreme vulnerability to drought-induced cavitation) contributing to an apparent extreme sensitivity to dry conditions. The results showed that capillary forces can drive refilling of the embolised xylem vessels, and provided the first in vivo visualisation of refilling in primary xylem tissue. These results are of fundamental importance in understanding the role of persistent primary xylem in the functioning of complex hydraulic networks in plants ranging in morphology from herbs to trees.

This project has been examining the vulnerability of plants to recent and future climate change. In particular, we have been focusing on the vulnerability to water stress. In the laboratory we have been examining plants from Macquarie Island (and our contrast localities in the Sth American Andes) to document their functional characteristics. We conducted a very short field season on Macquarie Island (5 days) in preparation for wind tunnel experiments that will seek to model water loss from key species and scale it up to landscape and island levels.

Hydraulic structures of plants play important roles in tolerance of temperature extremes and drought, two factors likely to increase with climate change. Studies are underway to determine how hydraulic traits of selected sub-Antarctic plant species may enhance or constrain acclimation to changing climatic conditions, thereby contributing to climate-driven change in vegetation composition. Detailed analyses of the cushion plant, Azorella macquariensis, revealed physiological attributes consistent with adaptation to a cold, wet, cloudy environment that make it vulnerable to conditions induced by climate change. Complementary eco-physiological studies on another sub-Antarctic cushion plant, Azorella selago, and other closely related species in the Chilean Andes have enhanced understanding of relationships between structure and function along complex gradients in temperature and rainfall, and provided further insight into the die-back of Azorella macquariensis. Our studies indicate that inability to respond effectively to recent complex climate change is the most likely primary cause of dieback, assisted with potentially secondary impact from pathogens.

Major changes are occurring in mid to high altitude vegetation on Macquarie Island where widespread death of plants has been observed over the last 3 years. It is of critical importance to understand this signal from an island sentinel of climate change. Accordingly, plant samples were collected during the 2012-2013 field season and analyses were begun to clarify processes across scales from biophysics to evolutionary ecology. In particular we are focusing on which hydraulic traits of selected subantarctic species may constrain their ability to adjust to changing climatic conditions, thereby contributing to climate-driven change in vegetation composition. Achieving this understanding is fundamental to identifying species that are most at risk from climate change, developing predictive models of vegetation change, and then contibuting to plans for management of natural resources under changing climate scenarios.